From Slow Cooker Revolution by America's Test Kitchen

"This appealing French vegetable dish featuring eggplant, zucchini, and tomatoes hails from Provence and is great to have on hand because it can be eaten hot, cold, or at room temperature, tossed with pasta or couscous, served over polenta, or as an accompaniment to roasted meats. Traditionally, however, it is a labor of love, with many recipes demanding that each vegetable be cooked separately for the perfect texture. We found that the slow cooker eliminated much of this fussy preparation, though to ensure deep flavor after hours in the slow cooker we lightly browned the vegetables and aromatics in a skillet first. We added flour here as well so it would thicken the sauce, since the juices from the vegetables would not have a chance to evaporate in the slow cooker. Since the flavor of fresh tomatoes was muted after slow-cooking we opted to use canned diced instead, the flavor of which was lasting and sweet. A sprinkling of fresh Parmesan cheese and fresh basil help round out this simple dish.

Transfer to slow cooker. Working in two batches, repeat with 2 tablespoons more oil and zucchini; transfer each batch to slow cooker.

Heat remaining 2 tablespoons oil in skillet over medium-high heat until shimmering. Add onions, bell peppers, garlic, and thyme and cook until vegetables are softened and lightly browned, 8 to 10 minutes. Stir in flour and cook for 1 minute. Slowly whisk in reserved tomato juice, scraping up any browned bits and smoothing out any lumps; transfer to slow cooker.

Stir tomatoes into slow cooker. Cover and cook until vegetables are tender, 4 to 6 hours on low. Stir in basil, season with salt and pepper to taste, and serve with Parmesan. (This dish does not hold well on warm setting.)